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Palms flourished in the Arctic during a brief sweltering period about 50 million years ago, according to a study that hints at gaps in our understanding of modern climate change.

The Arctic "would have looked very similar to the vegetation we now see in Florida," says Dr Appy Sluijs of Utrecht University in the Netherlands who led the international study. Evidence of palms has never been found so far north before.

The scientists, sampling sediments on a ridge on the seabed about 500 kilometres from the North Pole and up to 53.5 million years old, found pollen from ancient palms as well as of conifers, oaks, pecans and other trees.

"The presence of palm pollen implies that coldest month mean temperatures over the Arctic land masses were no less than 8°C", the scientists, based in the Netherlands and Germany, write in the journal Nature Geoscience.

That contradicts computer model simulations, also used to predict future temperatures, that suggest winter temperatures were below freezing even in the unexplained hothouse period that lasted between 50,000 and 200,000 years ago during the Eocene epoch.

Climate surprises

Sluijs says that it was also striking that palms, which do not lose their leaves in winter, grew in an area where the sun does not shine for about five months. Experiments with modern palms indicate that they can survive prolonged darkness.

The scientists say the presence of palms, it was not clear if they were trees or plants, hinted that the modern climate system could yield big surprises.

In 2007, Arctic ice shrank to its smallest size since satellite measurements began in the 1970s.

One possibility for the ancient spike in temperatures was an abrupt rise in carbon dioxide levels, far beyond current concentrations. That might have been caused by volcanic eruptions, or a melt of frozen methane trapped in the seabed.

"We cannot explain this with the current knowledge of the climate system," says Sluijs. One possibility was that new types of clouds formed in the Arctic as it warmed, acting as a blanket that trapped ever more heat and accelerated warming.

"If the ocean was very warm it's possible that these clouds form at a higher latitude than now," he says. Such effects caused by new cloud formation could be an unexpected tripwire in accelerating modern climate change.